Topic 1.3 Membrane Structure

Question 1

What is the Fluid Mosaic Model

Answer 1

1. Lipids (phospholipids, cholesterol)
2. Proteins (extrinsic/peripheral and intrinsic/integral)
3. Carbohydrates (glycolipids, glycoproteins)

Question 2

What is the structure of a phospholipid

Answer 2

1. Polar phosphate end is hydrophilic,
2. Hydrocarbon ends with 2 fatty acid tails is hydrophobic
3. Hence, the phospholipid is amphipathic
4. Can form monolayers, liposomes, micelles, bilayers

Question 3

Factors affecting the Fluidity of Membrane

Answer 3

1. Saturation
2. Length of fatty acid tails
3. Temperature
4. Cholesterol

Question 4

How does Saturation of fatty acid tails affect fluidity of membrane

Answer 4

More unsaturated = More double bonds = more kinks = prevent tight packing = decreased number of hydrophobic interactions = greater membrane fluidity.

Question 5

How does length of fatty acid tails affect the fluidity of the cell membrane

Answer 5

Shorter fatty acid tails = decreased number of hydrophobic interactions = more fluid

Question 6

How does temperature affect the fluidity of the membrane

Answer 6

1. Temp decreases, and membrane fluidity decreases.

2. The temperature at which membrane solidifies – phase transition temperature

Question 7

How does cholesterol affect the fluidity of the membrane

Answer 7

At Low Temp:
- Increases fluidity at low temp by disrupting close and regular packing of phospholipids –> preventing solidification of membrane.

At High Temp:
- Decreases fluidity at high temps by restraining movement of phospholipids –> prevent membrane from disintegrating.

Question 8

Structure of Cholesterol

Answer 8

• Amphipathic (polar OH group but other than that its non-polar)
• Head is attracted to phosphate heads of phospholipids, its tails to the tails of the phospholipids
• Synthesized in liver
• 4 fused rings
• Rarely in plant cells
• Helps membranes to curve into a concave shape.

Question 9

Why is maintaining membrane fluidity important?

Answer 9

Membrane fluidity must be carefully controlled.

• Too fluid – unable to control the passage of substances
• Not fluid enough – movement of the cell and substance within it is restricted.

Question 10

What are Extrinsic Proteins

Answer 10

Location:
Attached to the surface (inner-cytoplasmic, outer-extracellular),

Bonds:
- Hydrogen and ionic bonds between polar regions and polar head of phospholipids.

Question 11

What are Intrinsic Proteins

Answer 11

Location and Structure:

• Embedded in Hydrocarbon chains of the membrane (hydrophobic on at least part of the surface)
• Hydrophobic and hydrophilic on a different parts of the surface

Bonds:

1. Polar: Hydrogen and ionic bonds between polar regions and polar head of phospholipids
2. Non-Polar: Weak hydrophobic interactions btw non-polar protein and hydrophobic core of Phospholipids

Types of Intrinsic Proteins

1. Integral Unilateral Protein
   - Does not span across the entire membrane
2. Integral Transmembrane Protein
   - Spans across the entire membrane

Question 12

Types of membrane proteins

Answer 12

1. Receptors
2. Immobilized enzymes
3. Cell adhesion
4. Cell to cell communication and cell recognition
5. Transport
   - Passive
   - Carrier
   - Channel
   - Pump

Question 13

Function of receptor membrane proteins

Answer 13

o Recognize and bind with specific molecules outside the cell
o Integral proteins with a binding site complementary to shape of specific ligand.
o Upon binding, receptors are activated, stimulates cellular response by signal transduction pathways

Question 14

Function of immobilized enzymes

Answer 14

Integral proteins with active sites exposed to substrates in adjacent solution.

Examples:

1. Immobilized digestive enzymes bound on microvilli
2. ATPase inner membrane of mitochondria/thylakoid membrane

Question 15

Function of cell to cell communication and cell recognition

Answer 15

1. Glycoproteins serve as identification tags specifically recognized by membrane proteins of other cells.
2. Short live cell-cell binding

Question 16

Function of cell adhesion

Answer 16

o Membrane proteins of adjacent cells hook together in various kinds of junctions (e.g. tight/gap)
o Long lasting, virtually impermeable barrier to fluid and dissolved substances

Question 17

Function of channel proteins

Answer 17

o Transmembrane with hydrophilic pore

o Passive movement of ions and hydrophilic molecules across the membrane

Question 18

Function of carrier proteins

Answer 18

Binding of substrate = change in conformation = translocation across membrane

Question 19

Function of pumps

Answer 19

Same as carrier proteins but hydrolyses ATP for energy to transport substance against conc gradient.

Question 20

What is the sandwich model

Answer 20

Suggested a bilayer of phospholipids in the centre of membrane with layers of protein on either side.

• Evidence shows that the membrane has enough phospholipids to make twice as large area of membrane –> phospholipid bilayer
• Membrane form a barrier to transport some substance, suggesting layers of protein acting as the barrier

Question 21

Problems with the sandwich model

Answer 21

1. Amount and type of membrane protein varied among different cells
2. Did not account that membrane proteins are mainly hydrophobic, hence the majority of the surface area should not be facing an extracellular/cytoplasmic environment
3. Attempts to extract protein indicated that some occurred on the external surface but others were buried within or across the lipid bilayer and were harder to extract.
4. Freeze etching studies of plasma membrane show that some protein are seen to occur buried within or across the lipid bilayers.
5. Specific components “tagged’ by fluorescent dyes showed component molecules to be continually on the move within the membrane.

Question 22

Why fluid mosaic model is correct

Answer 22

1. Membrane is a phospholipid bilayer which is free to move about laterally, with a mosaic-like, random distribution of proteins in, or loosely attached to the phospholipid bilayer.
2. Proved by freeze fracture (freeze sample, cut with microtome knife to split the cell and expose the membrane’s layered structure), and electron micrographs. Showed that globular proteins embedded in the membrane with the hydrophilic regions protruding.
3. Found that there were both integral and peripheral proteins in the phospholipid bilayer, hence its not sandwich.